1. Field of the Invention
The present invention relates to a power supply system for parallel operation of a plurality of alternate current output converters, such as inverters, connected with a common load, for controlling balance of currents between converters.
2. Description of the Prior Art
FIG. 1 is a diagram showing a prior art parallel operating system for A.C. output converters as disclosed, for example, in Japanese Patent Publications No. 53-36137 and 56-1310.
Referring to the diagram, a first inverter 1 is operated in parallel with a second inverter 2 of the same structure connected through an output bus 3 for supplying power to a load 4. The first inverter 1 is structured chiefly of an inverter body 100, a filter reactor 102, and a filter capacitor 103, connected with the output bus 3 through an output switch 104, and is adapted to convert power from a D.C. power source 5 into A.C. power. In order to achieve parallel operation of the inverters 1 and 2, a detected signal I.sub.1a is first obtained by a current transformer CT 106 from the output current I.sub.1 of the first inverter 1. Then, a difference signal I between a detected signal I.sub.2a similarly obtained from the second inverter 2 and the detected signal I.sub.1a, corresponding to the cross current is obtained by a cross-current detector means 107. Thereupon, two voltage vectors E.sub.A and E.sub.B crossing each other at right angles are formed by a phase shifter 108, and these and the signal .DELTA.I are operated upon in arithmetic circuits 109 and 110 and a reactive power component .DELTA.Q and an effective power component AP are obtained, respectively. The inverter performs a pulse width modulation (PWM) for the inverter body 100 by means of a voltage control circuit 113 and a pulse width modulation circuit 114 based on output signals from a voltage setting circuit 111 and a voltage feedback circuit 112 and thereby controls the internal produced voltage.
Meanwhile, the reactive power component .DELTA.Q is supplied to the voltage control circuit 113 as an auxiliary. The voltage control circuit 113, in response thereto, adjusts the internal produced voltage of the inverter body 100 several % thereby reducing the reactive power component .DELTA.Q to zero.
On the other hand, the effective power component .DELTA.P is supplied through an amplifier 115 forming a PLL circuit to a reference oscillator 105 to finely adjust its frequency, and hence, the amplifier 115 and the reference oscillator 105 function so as to control the phase of the internal produced voltage of the inverter body 100 thereby reducing the component .DELTA.P to zero.
Thus, by controlling the voltage and phase to reduce .DELTA.Q and .DELTA.P to zero, the cross current between two inverters are eliminated and stabilized sharing of the load between them is attained.
However, there have been problems with the above described prior art system as follows. A first problem is that, when one of the inverters in parallel operation is faulty and thereby the output voltage becomes extremely low or high, it is possible for an excessive cross-current flow through another functioning inverter and thereby the functioning inverter also breaks down.
A second problem is that, since average values of the phase and the voltage of the internal produced voltage of the inverter are controlled to control the current for the load shared between the inverters, the allotted share of the current being thus controlled indirectly, it is difficult to improve the responding speed or accuracy of the controlling operation, and especially, it is impossible to control instantaneous allotment of the current.
A third problem is that the control of the effective current and that of the reactive current are not performed independently but they interfere with each other, and to avoid this interference, the response speed of the control cannot be raised so much.
A fourth problem is that, in a three phase system, when the reactance values of the reactors of the filters of the three phases are not equal but different from each other, the ratios of the allotted currents become different between phases.
The prior art parallel operating system for converters has been structured as described above and involves those problems as mentioned above, of which, it has been a primary problem that, when a breakdown occurs on one of the inverters in parallel operation and thereby the voltage on the broken inverter becomes extremely low or high, an excessive cross-current flows through another normal inverter and thereby this normal inverter also gets out of order.